An intravascular pressure-diameter gauge will be used to obtain pressure-diameter relationship curves for major pulmonary vessels in intact anesthetized dogs. These curves will be used to quantitate vascular smooth muscle responses from changes in vessel diameter and distensibility modulus. The effects of interventions, e.g., changes in blood volume, posture, blood gas concentration, etc. will be studied. Neural and hormonal mechanisms involved in smooth muscle responses will be identified using approaches such as selective autonomic blocking agents and blocking of nerves. The effects of vasoactive agents will also be investigated in terms of dose-response curves. In other experiments, the effect of pulmonary smooth muscle responses on the characteristic impedance, Rc, of the pulmonary artery will be quantitated. Rc will be obtained in the time domain rather than in the frequency domain. In this approach, Rc will be obtained as the ratio of the initial slope of the systolic arterial pressure rise over the initial slope of the flow pulse. Previous experiments showed that both the frequency domain and time domain methods agree well. The latter method, however, does not require steady-state conditions. Thus, pulse-by-pulse reflex changes in Rc may be studied as another way to elucidate pulmonary smooth muscle responses.